The subjunctive is a mood with past, present, and future tenses. It has two major roles in Latin, and you should try to keep these roles distinct in your head. Its first role is to denote an action that is wished, debated, unreal, or ordered: “if only I were taller,”, “what should I do?”, “if only you had come,” “let’s go.” This last example is a type of imperative called the jussive subjunctive. You’ve seen it in fiat lux in lesson two.
The other major role of the subjunctive in Latin is in subordinate clauses. The difference between co-ordinate clauses and sub-ordinate clauses is the difference, in English, between “I walk and I talk and I run” and “Since I had walked for ages, and although I wanted to run, I didn’t.” The since and although clauses in the second example are said to be subordinate to the main clause. This is distinguished by its main verb (“I didn’t”).
It’s a general rule of thumb that, in Latin, all verbs in subordinate clauses are in the subjunctive mood. This role is purely grammatical, and subjunctives of this kind should be translated, except in certain cases, as if they were in the indicative mood.